14 November 2020

I think, most of us agree that pollination is an important ecosystem service. But is pollinator limitation a thing? Is pollination a limiting factor for crop production? Or are other factors like nutrients or crop pests more important for reduced crop yields? This isn’t easy to answer, the weight of different factors will be different depending on the conditions.

Some years ago, I did a study in the cider apple orchards of Herefordshire in the UK. One of the aims was to monitor the bee communities in different orchards. Most of them conventionally managed, but some of them also organic. The differences between the bee communities depended not that much on the management, but on the structures within and around the orchards. The site where we found the most diverse and abundant bee communities was a beautiful orchard surrounded by hedges. At one side, there was a little forest. In between the hedge and between the rows there were old oaks and other trees. At one place, there was a large nesting association of two species of mining bees, one of the largest I’ve ever seen. There were also many nests between the trees and on the meadow between the rows.

On the other hand, we found almost no bees in another orchard. It was almost completely bare, the only thing growing there were the poor apple trees. Both orchards were conventionally managed, had almost the same pesticide regime. The main difference: the structures, the habitat for bees. And, of course, the growers’ approach. Apples are highly dependent on insect pollination; both of them knew that. The first said, “I do what I can to help nature.” The other asked a friend to bring some honey bee colonies during the flowering period.

Pollinator limitation and consequences for crop production

I had to think of this study when I read a recent paper that studied pollinator limitation in different crops in the USA. Our study in the UK didn’t look at fruit set or yield quality. However, it was one of the elements of nowadays letting me insist so much on the importance of habitat. However, Reilly et al. studied seven crops on 131 commercially managed fields in the USA. Their questions were:

  1. How frequent is pollinator limitation?
  2. What is the relative contribution of honey bees and wild bees to pollination?
  3. How do these contributions translate in economic value?

This connection between ecosystem service (pollination) and a specific economic value from managed and wild pollinators isn’t that frequent. Another interesting finding was the different proportion of pollination by wild bees not only between the crops but also between the sites. Wild bees, for instance, pollinated watermelons more often in Florida than in California. Wild bees weren’t the same at all places: Most wild bee visitors of sweet cherry in Washington were bumblebees. In the eastern USA, they found mainly solitary bees from the genus Andrena (mining bees).

This confirms something quite important: one pollinator species isn’t enough. Because the same plant could depend mainly on one group of pollinators at one place an on another at the other. And most importantly, honey bees didn’t do the largest part of the job. It depended again on the crop and the region. However, in apple, the crop I studied in the UK, they saw the same thing: most apple pollinators were wild bees.

The economic value of wild bee pollination varied in different crops. This doesn’t reflect the visitation alone, but also the value of the crop: I assume the there is much more apple than sour cherry production in the USA.

The economic contribution of wild bees to crop pollination

Apple was actually the crop in which wild bees contributed most to the economic value of the crop. As I mentioned above, this crop is highly dependent on insect pollination. If the quality of the pollination isn’t good enough, there’s a lower yield, but also the quality of the fruit is lower. To be fully marketable apples must be round and spotless. Every little flaw decreases their value – consumers don’t want less than perfect fruit. This is totally wrong and adds to climate change and many other issues, but let’s stay with the pollination today. In this study, at the specific sites monitored, the value of wild bee pollination was over a billion US dollars. Let that sink in for a second.

What surprised me quite a bit, however, that the value of wild bees in almond in this study was none. The very first contact I had with crop pollination was during my master thesis. It was about the management of Osmia cornuta, a mason bee, for almond pollination. The authors justify this result with the conditions of almond production in the USA, which is completely different from those in Europe. Plus, in the States, transporting thousands of honey bee colonies all over the country for almond pollination is an important income for beekeepers. And the orchards look much more… well, industrial than they do in Europe.

Again: no habitat for wild bees. An indicator in the study from Reilly and colleagues was that they found more wild bees in small scale farms surrounded by natural habitat than at the larger farms. Do you recognize the principles we talked about last week?

Pollinator limitation and wild bee contribution

Another difference between this study and European conditions may relate to pumpkin: in the USA there are squash bees, species that are specialists for pumpkin and squash pollen. We don’t have these here in Europe. In our garden, I saw mainly honey bees in the pumpkin flowers. In the study, wild bees were the majority of the visitors.

A common principle, however, was again that wild bees were more important for early blooming crops. These were also more susceptible to pollinator limitation. This confirms findings from other studies: honey bees like it quite warm. They fly at higher temperatures than bumblebees or some solitary bee species that are active already in early spring. While in the UK, we found mostly different mining bee species in apple flowers. The grower with the orchard we found the least wild bees in was the most desperate for the insufficient pollination of his crop. Though he brought in honey bees, this didn’t have consequences on the flowers. It was too cool, the foragers from those colonies weren’t active enough to do the job. And to be honest: the colonies didn’t look very good…

However, on six of the seven crops (i.e. all except almond), wild bees deposited more pollen than honey bees. That means that they were more efficient pollinators than their managed cousin was. In apple, for example, this results in wobbly, irregular fruit with less value.

If not enough pollen arrives on the stamp of an apple flower, the apple becomes wobbly like this. The grower can’t sell this as an table apple anymore, it gets into juice production. Which means a lower price.

Solutions for pollinator limitation

The authors of this study found that pollinators were a limitation in five of the seven studied crops. With the expected consequences. Despite the conditions of North American or European agriculture are quite different, I think there’s again a common principle here. Pollinator limitation IS a thing; it just may not be studied that systematically in other parts of the world. We do have plenty of indicators for this, though. I totally agree on the authors’ conclusion that the variability of the bee fauna visiting different crops makes it urgent to study pollinators in different regions.

Their opinion that pollination may be more limited nowadays than in the past is another thing I can completely share. I agree less on the solutions they discuss to solve the problem of pollinator limitation in commercial crops. Using alternative managed pollinators and higher stocking rates with honey bee colonies near the crops. It kind of contradicts their own finding that wild bees were important pollinators. In addition, managed pollinators have their own set of problems they put to bee health. If not managed carefully, they transmit diseases to wild pollinators. By this, they could add to the problem instead of solving it.

In general, I’m not a big fan of looking only for technical solutions for this issue. Managed pollinators are complementary to wild pollinators. And to have more of the latter doing their job in food crops we need mainly one thing: habitat, habitat, habitat. Which, by the way, is something I offer consulting services for.

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